One known issue with Dynamic or Active Battery Cell Balancing is that its effectiveness is compromised by variations in temperature across the pack. Battery cell temperatures vary due to differences in individual chemical compositions and internal resistances. If the individual state of charge (SOC) estimation is not made at the same thermal potential then the charge balancing system is not able to make accurate judgments on the need to transfer charge.
If the pack is assembled with a technical grade wax jacket surrounding each battery cell then the temperature can be equalized to be homogenous across the pack. The wax acts as a phase change material, rapidly melting at a relatively low temperature and thus able to transfer heat by natural convection between the battery cells.
A weakness and source of premature failure in current multi-cell battery packs is the tab bonding used to interconnect the battery cells. The tabs are spot-welded in a process which injects heat locally, an undesirable action which promotes premature failure. Furthermore the bonds have variable impedances which affect both the absolute performance of the pack and the effectiveness of the battery management systems.
Alternatively if a wiring harness is employed for interconnect and battery management purposes, this too is a source of potential failure by adding a multiplicity of connection nodes of varying impedance and each with the potential for outright failure or compromising the effectiveness of the battery management system. Wiring harnesses add cost and complexity and are subject to the same quality issues affecting any manually intensive process.
Ultrasonic wire or ribbon bonding is a technique used successfully in the semiconductor industry over a wide range of products including high power devices capable of handling similar currents to those found in light electric vehicle applications. By targeting this technology at the battery pack a high degree of automation is possible and a consequent improvement in reliability can be demonstrated. Connection points to the battery cells need to be migrated to a single end for ease of access and assembly.